I used it for an open-ended project/event where soemtimes the tasks became mini-projects in their own right. That's it's power - you can make a task a checklist of sub-tasks very easily without "promoting" it or dealing with a tree interface.

Also the uploads and notes are very easy to absorb and view. Delegation also looks great... though I haven't had a need to use or see how it integrates with email.

I used it for trip planning, meeting planning and personal checklists. However since it's hosted and it's unclear what the data security is like, I don't use it for work related stuff.

Mostly, it's just very very easy to start using and easy on the eyes. I'm drawn to finding uses for it, though I keep wanting calendar/scheduling support (which would make it more of a full-on process/group ware)